Current-sense amplification is used in a variety of different applications. For example, memory circuits that are able to store a bit of information output a relatively high current if the bit is logic one, and a relatively low current if the bit is logic zero. Current-sense amplification is used to effectively amplify these currents, so that it is easier to discern between a logic one and a logic zero stored within a memory circuit. Such memory circuits include those in which memory information is conveyed by a change in transistor drain current, and/or where the transistor potential remains at a low level. Such memory circuits also include magnetic tunnel junction (MTJ)-magnetoresistive random access memory (MRAM), MTJ-MRAM: tunneling magnetoresistive (TMR), as well as other types of memory circuits, such as some types of programmable read-only memory (PROM).
FIGS. 1A and 1B show conventional sense amplifiers, according to the prior art. More specifically, FIG. 1A shows a differential current-sense amplifier that uses a current mirror load, whereas FIG. 1B shows a cross-coupled sense amplifier that is commonly used for dynamic random access memory (DRAM). In FIG. 1A, the signal to be sensed is input at terminal IN, and the opposite of the signal to be sensed is input at terminal INN. The output of the signal to be sensed is provided at terminal OUT, and is high when the signal is high, and low when the signal is low. In FIG. 1B, the signal to be sensed is input at terminal DIO, and the opposite of the signal to be sensed is input at terminal DION. If the signal is relatively high, the terminal DIO is amplified to the power supply potential, and otherwise to the ground potential. If the signal is relatively low, the terminal DION is amplified to the power supply potential, and otherwise to the ground potential.
In FIG. 1A, the output of the differential current-sense amplifier never reaches logic level. As a result, to obtain a logic level output, a backend amplifier is required, and circuits, such as a bias circuit, for the configuration and control of a constant current source are also required. Furthermore, the differential current-sense amplifier of FIG. 1A is asymmetric, and therefore to obtain symmetric output, it is common to arrange two instances of this amplifier in parallel. Finally, current flows through the amplifier at all times, and also the control circuit which generates a potential, or voltage, to be provided to the device used as the constant current source in the amplifier consumes direct current (DC).
In both FIGS. 1A and 1B, due to the threshold voltage of the transistors that have to be used, input potential cannot have a value that is close to the potential of the power source itself or the ground potential. When current is sensed, by adding a resistor and applying an input current to the resistor, the terminal voltage of the resistor is used for current sensing. In this situation, the terminal voltage of the resistor has to be raised to an intermediate potential. As a result, the potential of a memory device being sensed will be halfway between the potential of the power source and the ground potential. This is problematic for memory devices in which potential cannot be raised to extract a current, such as MTJ-MRAM devices and the memory devices for PROM's, for instance. For these and other reasons, therefore, there is a need for the present invention.